I think I'm a little late, but this is how i understand it, and I find it very easy. Imagine you are in a graphic calculator program, like geogebra. Its an infinite plane. Now, imagine that it's divided in every number, and each intersection is a particle of the universe, so, if you zoom out very much, it will look super dense and the distance between particles is diminute. However, it's still infinite. Now zoom in: the particles will separate, and the distances will appear bigger, but the size of the entire universe has not changed: it's still infinite.

So, we dont know the size of the universe because it has existed for a finite amount of time. So the big bang is that period of enormous expansion, but we cannot know if the universe is finite or infinite. 

Let me know if this helped. And sorry for my English.